The present invention generally relates to ventilator systems that are used in assisting the respiration of a patient to treat disturbed breathing, snoring, mixed obstructive sleep apnea, and certain cardiovascular sleep conditions. More particularly, the present invention pertains to the control of the air pressure to which the patient is subjected to by the ventilator during each respiratory cycle to provide a system well-suited for homecare applications.
Obstructive sleep apnea is a sleep disorder characterized by the relaxation of the airway, including the upper airway muscle tissue during sleep. When this occurs, the relaxed muscles can partially or completely block the patient's airway, a condition more prevalent in overweight patients. Partial blockage can result in snoring while complete blockage can result in sleep apnea. When complete blockage occurs, the patient's ventilation efforts do not result in the intake of air and the patient becomes oxygen deprived. In reaction, the patient begins to awaken and upon reaching a nearly awakened state, the upper airway muscles resume normal tension which clears the airway and allows inhalation to occur. The patient then falls back to the deeper sleep whereupon the upper airway muscles again relax and the apneic cycle repeats. Central apnea is a condition wherein no inspiratory effort occurs or is delayed. Both central apnea as well as obstructive sleep apnea may be as present simultaneously, a condition referred to as mixed apnea. Other breathing irregularities are known which involve apneic intervals, Cheyne-Stokes breathing, being an example thereof.
In some patients, sleep apnea events can occur hundreds of times during a sleep session. As a consequence of the repetitive arousal to the nearly awakened state, the patient never achieves fully relaxed deep sleep and is deprived of REM (rapid eye movement) sleep. Additionally, the patient's blood oxygen falls to subnormal levels. People afflicted with sleep apnea are continually tired even after an apparently normal night's sleep, while the continual or repeated oxygen depravation may have an adverse affect on the patient's cardiovascular system.
In order to treat obstructive sleep apnea, so-called continuous positive airway pressure (CPAP) systems have been devised in which prescribed levels of positive airway pressure are continuously imposed on the patient's airway. The presence of such positive pressures within the airway provides a pressure splint to offset the negative inspiratory pressure thereby maintaining tissue in position and the patient's airway open. The positive pressure is typically generated by a blower, the output of which is ducted to the patient and connected to the airway by a nasal pillow which seals with the patient's nares. Control valves in the system control the pressure to which the patient's airway is subjected.
In prescribing the CPAP therapy, it is usually necessary for a patient to spend one or two nights in a sleep treatment laboratory where it is first determined whether the patient has a respiratory disorder such as sleep apnea. If so, the patient is then fitted with a CPAP device whereupon pressure and volume parameters are determined for providing the necessary air splint and satisfying the patient's respiratory requirements.
A number of shortcomings, are associated with the previously known CPAP systems. Two fundamentally different approaches have heretofore been taken with respect to the manner in which the breathing is controlled each suffering from a number of disadvantages. Initially, ventilator systems were designed to deliver a predetermined volume during the inspiration phase of each breathing cycle. While this approach positively ensures adequate respiration even for patients completely incapable of breathing on their own, the rigorous routine is perceived as quite uncomfortable by patients requiring less breathing assistance. The prescribed volume of air is after all forced into the patient's airways without regard to the pressures that may be generated and independent of what rate the patient would consider comfortable. Such systems are therefore not well matched to the needs of the homecare market, especially in the treatment of sleep disordered breathing, and are today reserved exclusively for very critical care applications.
Substantially more comfortable breathing assistance is provided by ventilator systems wherein the respiratory cycle is pressure driven. Such systems may be configured to supply air at a predetermined inspiratory positive airway pressure (IPAP) upon sensing the onset of inspiration and until the patient initiates exhalation. Upon exhalation, system pressure is immediately reduced to a predetermined expiratory positive airway pressure (EPAP) to facilitate the expulsion of air from the patient's airway. This type of system augments a patient's spontaneous tidal volumes and was the accepted mode for assisting the patient to overcome the work of breathing associated with an artificial airway, the mechanics of the ventilator and for the weaning of the patient from the full support of mechanical ventilation. In its simplest form, such system does not take into account the actual volume of air respirated by the patient. Consequently, despite satisfying the prescribed pressure parameters, the patient may nonetheless suffer from hypoventilation, reduced PAO.sub.2, reduction in daytime alertness and increased CO.sub.2 levels.
More recently, several hybrid forms of pressure support have been introduced which vary in method and adjustment but strive to overcome the problem associated with varying tidal volumes using pressure limited modes of ventilation. One way of accomplishing this is to vary the amount of pressure to achieve satisfactory gas exchange in light of changing compliance/resistance components by estimating or actually monitoring such parameters. Since such approach may still require fairly close monitoring of the patient in order to avoid hypoventilation and the consequences thereof, many of the corresponding devices are still better suited for use in hospitals rather than in the homecare environment. U.S. Pat. No. 5,134,995, which discloses a variety of systems that undertake to adjust air pressure to accommodate various conditions, is hereby incorporated by reference.
A system is needed that ensures adequate ventilation of a patient while eliminating the need for monitoring and supervision so as to provide a device suited for the homecare market. Previously known systems have been unable to adequately satisfy this need.